The present invention relates to a flexible metallic joint of the type comprising at least one elastic metal core having, when in the rest state, a toroidal shape, and at least two envelopes in which are fixed said metal core, each envelope being open along one of the peripheries of the metal core.
In metal joints of this type, the toroidal elastic core is either constituted by a metal tube or by a helical spring having contiguous turns and which is closed on itself. In the latter case, the cross-section of the wire constituting the spring can be of a random nature and particularly round, rectangular or in the form of a circular segment. The tube or helical spring, which is fixed on the site, gives the joint its elasticity.
The envelope surrounding the core of existing joints of this type are constituted by materials in plated or thin sheet form, such as polytetrafluoroethylene, aluminium, silver, copper, nickel, tantalum, stainless steel, zirconium, etc.
Each of the envelopes has a particular function. Thus, when the core of the joint is constituted by a spring with contiguous turns, the inner envelope generally serves to distribute the load at each turn apex of the elastic core. The outer envelope is made from a ductile metal which, due to its capacity to flow into the contacting surface unevennesses, gives the joint a perfect seal when locked into an assembly. In general terms, it is therefore the plastic properties of the materials forming them which characterize the envelopes of the joint.
It should also be noted that the existing metal joints generally have a toroidal shape whilst the envelopes gripping round the elastic core also generally have the shape of a toroidal surface, whose generating circle does not close on itself. An illustration of joints, whereof the elastic core is constituted by a helical spring with contiguous turns, is provided by French patent application No. 7,319,488, filed May 29th 1973 in the name of the Commissariat a l'Energie Atomique, now French Pat. No. 2,232,235.
Industrial experience has shown that these elastic metal joints are satisfactory for numerous applications. However, in the case where the assemblies are subject to differential radial displacements or movements between the flanges, the initially obtained seal can lose its effectiveness.
These differential movements are encountered whenever the flanges of the assembly are made from heterogeneous or anisotropic materials, or from homogeneous materials when the temperature rise or fall conditions are not homogeneous for the complete assembly. In practice, these conditions are very frequently encountered.
In order to analyse the behaviour of existing flexible metal joints under the aforementioned conditions, reference is made to FIG. 1 which, in cross-sectional form, shows an existing flexible metallic joint 10 secured between two flanges A and B.
More specifically, FIG. 1 shows in exemplified manner a joint 10, whose elastic core is constituted by a helical metal spring 12 having contiguous turns, said spring being fixed in an inner first envelope 14, which is itself fixed in an outer second envelope 16. References P.sub.1 and P.sub.2 designate the contact points of the outer envelope 16 of the joint 10 respectively with flanges A and B, after the assembly has been secured.
If it is assumed that flange A expands more than flange B following the securing of the assembly, point P.sub.1 of flange A passes to P'.sub.1. During this displacement, the outer envelope 16 of the joint 10 is stressed whereby it undergoes stretching and then sliding, because that part of the outer envelope 16 whose length is defined by points P.sub.1 and P.sub.2 is jammed against the helical metal spring 12 and cannot be subject to elongation without damaging the joint 10. This leads to a loss of sealing for the joint 10 and this can be increased if the phenomenon is repeated in one or other direction and with a variable amplitude throughout the life of the assembly.